Transformer Modeling Using Simulator

PowerWorld Program Users Work GroupAugust 23, 2011Presented by John Gross

Phone: 509-495-4591Email: john.gross@avistacorp.com

General Purpose and Operation

•Maintain near constant secondary voltage with a variable primary voltage•N1 and N2 are fixed turns ratios determined by connected positions (N2 can be changed with transformer out of service)•NT represents the LTC which adds or subtracts windings to the N1 turns ratio•Avista’s typical LTC on Autotransformers has 17 positions; range 247.5 kV to 225.5 kV with nominal at 236.5 kV

N2

N1V1

V2

+

- -

+

NT

222

11 VaV

NNNV T

Mechanical Operation

•Various methods/devices are used for LTC•Example at right: Tap selector switches numbered 1 to 8 Diverter resistors A and B prevent high

circulating current Change from tap position 2 to 3

– Close switch 3 under no load– Rotary switch causes load current to

flow through resistor A, then momentarily through both resistor A and B with switches 2 and 3 closed

– Once resistor B is shorted, load current flows through switch 3

– Switch 2 now opens under no load

SCADA vs. Modeling Labeling Convention

SCADA Convention•Positive tap position increasessecondary voltage, total tap ratio is decreased•Negative tap position decreasessecondary voltage, total tap ratio is increased

Modeling Convention•Positive tap position decreasessecondary voltage, total tap ratio is increased•Negative tap position increasessecondary voltage, total tap ratio is decreased

a = (236.5+1.375x-8)/112.75 = 2a = (236.5+1.375x+8)/112.75 = 2.195

Other Labeling Conventions

247.5 kV

242.0 kV

236.5 kV

231.0 kV

225.5 kV

1

5

9

13

17

Any labeling convention works so long as it is consistent and documented!

LTC Discussion

•LTC Advantages: Voltage control Added flexibility

•LTC Disadvantages: Costs: Adds ~15-18% Maintenance issue:

– Moving parts– Increased outages

Failure probability higher with LTC LTCs generally not utilized today (confirm with PI data)

LTC Discussion Cont.

•PJM has analyzed and recommends NO LTC on 500/230 kV banks•Alternatives to LTC’s: Capacitor banks: distribute MVAR to optimum location

– Approximately 3 tap positions is equivalent to one cap bank:• 22 MVAr at 115 kV or• 67.1 MVAr at 230 kV

– No flexibility for high voltages Move generation (not likely for Avista) Specify LTC’s on lower voltage transformers Specify LTC for special situations

Taps vs Caps

LTCInitial cost: ~$390k Assume 250 MVA transformer 15-18% of total transformer cost

Maintenace: $20k, 5-7 years 70,000 operations Requires outage coordination

Life expectancy: 50 yearsTotal life cycle costs: ~$500k

CapacitorsInitial cost: ~$750k 3 – 70 MVAr banks at 230 kV Land acquisition will add to cost

Maintenance: $20k, 5-6 years 4000 operations

Life expectancy: 14 years for CB Circuit breaker mechanism will require

parts: $300kTotal life cycle costs (50 years): ~$2 million

Where do you start?

Transformer test reports and other company records

Collect the following data:

1. Impedance (%Z and full load loss)

2. Primary nominal voltage

3. Secondary nominal voltage

4. MVA rating

5. NLTC fixed tap position

6. LTC min & max tap

7. LTC step size

What base values should you use?

System MVA base (100 MVA) & system voltage base

• i.e. 500 kV, 230 kV, 115 kV

System MVA base & transformer voltage base

Transformer MVA base & system voltage base

Transformer MVA base & transformer voltage base

• Read directly from test report

• Less calculations!

The Transformer Model

Let the software do the base conversion

Transformer Base Model

System Base Model

Example Test Report

Example Data Entry

Data from test report

Modeling Assumptions

Magnetizing impedance is neglected

• Magnetizing current ~ 0.5% of rated current

• Less than 20% xfmrs in WECC cases model B and G

Impedance is proportional to number of turns squared

• If NLTC is off nominal (i.e. + 2.44%), Z = Znom(1.0244)2

• Measured impedance may differ by ~ ±8% or ~ 0.5 ohms

Conversion to System Base

Let the program do the calculations

Conclusions

Hand Calculations should be avoided

• Computers excel at repetitive mathematical operations

• IF a bug exists that causes a math error when fixed all of the errors will be corrected

- This is NOT the case with hand calculations

Data needs to be easily traced back to a proper source

• If you enter data off the test sheets (as much as possible) then there is NO DOUBT as to the accuracy of the data.

Avista Procedure